Field of the Disclosure
Certain aspects of the present disclosure generally relate to motion compensation between water-borne objects, and, more particularly, to synchronizing motion between a remotely operated vehicle (ROV) and a docking station of a launch and recovery system (LARS).
Description of Related Art
Marine seismic surveys are one type of marine geophysical survey which utilizes sound waves transmitted to the earth's crust and reflected back to recording sensors. The recording sensors may be hydrophones or other sensors in one of a number of towing assemblies, commonly called streamers, that may be towed behind a survey boat. When towed behind the survey boat, the streamer may be submerged. A sound, or other energy, source may also be towed in the water behind the survey boat for transmitting energy waves to be received by the receivers of the streamers. One common application of marine geophysical surveying is oil and gas exploration in marine environments. More particularly, sound waves received during a marine seismic survey may be analyzed to locate hydrocarbon bearing geological structures, and thus determine where deposits of oil and natural gas may be located. In a similar fashion, marine electromagnetic (EM) surveys may be conducted using EM energy transmitted by a submerged antenna and detected by EM receivers.
Remotely operated vehicles (ROVs) may be useful for supporting marine geophysical surveying. For example, an ROV may be deployed to maintain (e.g., clean, repair) a streamer towed behind a survey boat, allowing maintenance of a streamer without reeling the streamer back onto the survey boat. ROVs may also be used for other tasks in marine exploration, such as placing equipment on the seabed.
ROVs may be deployed from a mother vessel equipped with a launch and recovery system (LARS). A LARS can be equipped with active heave compensation (AHC) to keep an ROV docking station, for example, a tether management system (TMS), of the mother vessel stable while a vessel (e.g., the mother vessel) coupled to the ROV docking station is subjected to motion, e.g., heave, roll, and/or pitch, etc. In some situations, launching and/or recovering an ROV, from the ROV docking station, takes place in deep water (e.g., 100 meters), away from the influence of free surface water waves. In general, the effects of wave particle motion and/or the wave pressure decay exponentially with water depth and thus, when launching and recovering an ROV in deep water, the vertical motion of the ROV induced by free surface waves can be neglected. The motion of the docking station induced by the effect the motion of the vessel has on the docking station can be counteracted, e.g., by AHC. Counteracting the effect the vessel has on the docking station can allow for improved launching and/or docking of the ROV.
Launching and recovery of an ROV, from the ROV docking station, is not always performed in deep water. In some situations (for example, when an ROV is being used to maintain a streamer being towed behind a survey boat), it may be advantageous to launch and recover an ROV close (e.g., less than ten meters) to the sea surface. In situations where launching and recovery (e.g., undocking and docking) takes place close to the sea surface, the motion of the ROV can be affected by particle motions and/or pressure fluctuations from the free surface waves. As an example, if the free surface waves are large in comparison to the docking depth, the motion of the ROV can be affected by wave particle motion and/or wave pressure. Motion of the ROV during launch and recovery operations can interfere with those operations, possibly damaging the ROV, docking station, and/or LARS. It is therefore desirable to counteract motion of an ROV during launch and recovery operations.